Gas turbine engine fans or compressors include a plurality of circumferentially spaced rotor blades which operate at rotational speeds which result in subsonic through supersonic flow of air over the blades thereof. Swept-back blades include a leading edge which is inclined rearwardly relative to the incoming air for reducing shock losses and noise generation at supersonic operation of the blade. Backward sweep is applied conventionally for both mechanical and aeroelastic considerations including being able to provide suitable structural strength at speed and acceptable levels of vibratory induced stress.
Although the swept-back rotor blade is effective for reducing shock losses, the blade tip sections are accordingly located further axially downstream from the pitch, or midspan, section of the blade. As a result, the boundary layer air which is centrifuged radially outwardly along the blade as the blade rotates collects adjacent to the blade tip, which is disposed radially inwardly from a conventional casing. The accumulation of this boundary layer at the blade tip interacts with the shock systems occurring at the blade tip which result in a decrease in aerodynamic performance. The blade tip boundary layer is formed in main by frictional, diffusion and tip clearance effects that tend to be enhanced by the aerodynamic effects brought on by backward sweep, and in part by the boundary layer centrifuged radially outwardly along the blade.
To incorporate the backward sweep into the leading edge of the blades, it is conventional to barrel the rotor blades by increasing the blade chords located adjacent to the pitch section of the blade resulting in a barrel-shaped blade. Barrelling the blade in its leading edge region results in the incoming air physically first encountering the leading edge at the pitch section. This in turn causes the air streamtubes to move radially inwardly from the blade tip, which is reflective of added diffusion requirements, which weakens the blade tip flow which in turn is detrimental to the overall performance of the blade row.